Conventional vehicular steering systems have an articulated mechanical linkage connecting an input device (e.g., steering wheel or hand-wheel) to a steering actuator (e.g., steerable road wheel). Even with power assisted steering in an automobile, for example, a typical hand-wheel motion directly corresponds to a resulting motion of the steerable road wheels, substantially unaffected by any assist torque.
However, for a vehicular steering system with active steering, such as that used in an automotive front-controlled steering system, a given motion of the hand-wheel may be supplemented by an additional motion, such as that from a differential steering actuator, which translates into a motion of the steerable road wheels that does not necessarily correspond to the given motion of the hand-wheel. Consequently, when the differential steering actuator is inactive, the motion of the steerable road wheels directly corresponds to the hand-wheel motion due to the articulated mechanical linkage, just as in conventional systems.
The term “active steering” relates to a vehicular control system, which generates an output that is added to or subtracted from the front steering angle, wherein the output is typically responsive to the yaw and/or lateral acceleration of the vehicle. It is known that, in some situations, an active steering control system may react more quickly and accurately than an average driver to correct transient handling instabilities. In addition, active steering can also provide for variable steering ratios in order to reduce driver fatigue while improving the feel and responsiveness of the vehicle. For example, at very low speeds, such as that which might be experienced in a parking situation, a relatively small rotation of the hand-wheel may be supplemented using an active steering system in order to provide an increased steering angle to the steerable road wheels.
Conventional hydraulic and electric power-assisted steering systems generally lack an active element for providing control over the steering angle independent of a driver's input. This is because prior attempts at adding such an active element have not integrated well with existing systems and/or create too much torque feedback to the driver during active steering control. Therefore, it can be appreciated that there is a need for a tunable assist capability integrated with active steering.